There is an increasing demand for mobile communication devices capable of operating across a variety of different frequency bands. Common examples of such devices include cellular phones that operate in multiple frequency bands. These devices typically employ transmit and receive filters to tune each transmit and receive frequency band.
Various types of acoustic resonators can be used to construct filters for appropriate applications. Examples of these acoustic resonators include bulk acoustic wave (BAW) resonators such as thin film bulk acoustic resonators (FBARs) and solid mounted resonators (SMRs). BAW resonators can also be employed to construct oscillators such as tunable voltage controlled oscillators (VCOs) for some applications.
A typical implementation of an acoustic resonator comprises a piezoelectric layer (e.g., a layer of piezoelectric material) disposed between two electrically-conductive (e.g., metal) electrodes.
FIG. 1 is a cross-sectional view of an acoustic resonator 10 comprising a piezoelectric layer 12 disposed between a bottom electrode 11 and a top electrode 13. The designations top electrode and bottom electrode are for convenience of explanation, and they do not represent any limitation with regard to the spatial arrangement, positioning, or orientation of acoustic resonator 10.
During operation, an electric field is applied between first electrode 11 and second electrode 13 of acoustic resonator 10. In response to this electrical field, the reciprocal or inverse piezoelectric effect causes acoustic resonator 10 to mechanically expand or contract depending on the polarization of the piezoelectric material, as indicated by an arrow in FIG. 1. As the electrical field varies over time, an acoustic wave is generated in piezoelectric layer 12, and the acoustic wave propagates through acoustic resonator 10. For example, in some implementations, the acoustic wave propagates in parallel with the electric field as a longitudinal wave, or along the mechanical interfaces of acoustic resonator 10 as a lateral wave.
For longitudinal waves, where a thickness d of piezoelectric layer 12 and of the top and bottom electrodes equals an odd (1, 3, 5 . . . ) integer multiple of half the wavelength λ of the acoustic waves, resonance states and/or acoustic resonance vibrations will occur. Because each acoustic material has a different propagation velocity for the acoustic wave, the fundamental resonance frequency, i.e. the lowest resonance frequency FRES, will then be inversely proportional to a weighted sum of all thicknesses of the resonator layers.
The piezoelectric properties and, thus, also the resonance properties of an acoustic resonator depend on various factors, e.g. on the piezoelectric material, the production method, the polarization impressed upon the piezoelectric material during manufacturing, and the size of the crystals.
FIG. 2 is a graph illustrating a logarithmic input impedance response versus frequency for an example acoustic resonator. As shown in FIG. 2, the input impedance of the example acoustic resonator exhibits a sharp negative-going peak from a series resonance at a lower frequency fS, and a sharp positive-going peak from a parallel resonance at a higher frequency fP.
FIGS. 3A through 3C are circuit diagrams illustrating electrical models of a BAW resonator such as an FBAR. The model of FIG. 3A is a modified Butterworth-Van Dyke model (MBVD) model. The frequency response of this model is a bandpass response, with frequencies below the passband being attenuated by capacitances Cm and Co, and with frequencies above the passband being attenuated by an inductance Lm. As shown in FIG. 3B, at series resistance, the BAW resonator can be modeled by a series-resonant combination of inductance Lm and capacitance Cm in series with a parasitic resistance Rs. As shown in FIG. 3C, at parallel resonance, the BAW resonator can be modeled by a parallel-resonant combination of inductance Lm and capacitance Co in parallel with a parasitic resistance Rp. Resistances Rs and Rp represent various heat losses and acoustic losses within the acoustic resonator.
An acoustic resonator can be employed in various types of electrical filters, such as radio frequency (RF) filters or a microwave filters. In addition, acoustic resonators can be combined in various ways to produce a variety of filter configurations. The performance of an RF or microwave filter constructed with an acoustic resonator depends on the performance of the acoustic resonator, which can be expressed in terms of the resonator's parallel resistance Rp, series resistance Rs and its electromechanical coupling coefficient kt2. Referring to FIG. 2, the series resistance Rs is the smallest value of magnitude of input impedance, and series resonance frequency Fs is a frequency at which that minimum occurs. The parallel resistance Rp is the largest value of magnitude of input impedance, and parallel resonance frequency Fp is a frequency at which that maximum occurs. The electromechanical coupling coefficient kt2 is a normalized difference between parallel and series resonance frequencies Fp and Fs and is typically expressed in percent values (%). In general, devices with higher Rp or kt2 and lower Rs are considered to have superior performance than devices with higher Rs or lower Rp or lower kt2. Thus, other things being equal, it is desirable to provide a filter with an acoustic resonator having a higher Rp or kt2 and lower Rs.
An acoustic resonator can also be employed in an oscillator. Where an acoustic resonator is employed in an oscillator, the performance of the oscillator (e.g., phase noise) is affected by the Rp or kt2 of the acoustic resonator. Moreover, as with filters, it is also desirable to provide an oscillator with an acoustic resonator having a higher Rp or kt2 and lower Rs.
Unfortunately, many design choices that increase the Rp of an acoustic resonator tend to decrease the kt2 of the acoustic resonator, and vice versa. In other words, there is generally a tradeoff between Rp and kt2. Consequently, applications requiring high Rp may be required to sacrifice kt2, and applications requiring a high kt2 may be required to sacrifice Rp.
What is needed, therefore, are acoustic resonator structures that can provide appropriate values of Rp and electromechanical coupling coefficient kt2 according to the demands of different applications.